UPSC Prelims 2019 Q44: Geography – Atmosphere & Climatology
- (a) Clouds absorb the radiation released from the Earth's surface.
- (b) Clouds reflect back the Earth's radiation.
- (c) The Earth's surface would have low temperature on cloudy nights.
- (d) Clouds deflect the blowing wind to ground level.
Answer: (b) Clouds reflect back the Earth's radiation.
Explanation
Why No Dew on a Cloudy Night
| Option | Claim | Verdict | Detail |
|---|---|---|---|
| (a) | Clouds absorb Earth's radiation | Partially true but not the primary mechanism | Clouds do absorb some IR but the dominant effect is REFLECTION back to surface. |
| (b) | Clouds reflect back Earth's radiation | CORRECT | Cloud cover traps outgoing longwave (IR) radiation and re-radiates it back to the surface - Earth's surface stays WARMER on cloudy nights. Since the surface does not cool below the dew point, dew does not form. |
| (c) | Earth's surface has LOW temperature on cloudy nights | FALSE (reversed) | Actually surface stays WARMER, not colder, on cloudy nights (that's why dew doesn't form). |
| (d) | Clouds deflect wind to ground | Irrelevant | Wind deflection is unrelated to dew formation. |
Answer: (b) Clouds reflect back the Earth's radiation.
Why each option is right or wrong
- (a) Incorrect. Absorption plays a small role - the dominant effect is reflection/re-radiation.
- (b) CORRECT. On cloudy nights, clouds trap and reflect outgoing longwave radiation back to Earth's surface. The surface stays warmer, does not cool to the dew point, so dewdrops do not form.
- (c) Incorrect. And reversed. Cloudy nights are WARMER, not colder. It is clear nights that cool most and produce dew.
- (d) Incorrect. Wind deflection has nothing to do with dew formation.
Key point
No dew on cloudy nights because clouds reflect outgoing longwave radiation -> surface stays warm -> doesn't reach dew point -> no condensation.
Remember
Dewdrops do not form on cloudy nights because clouds reflect back the Earth's outgoing longwave radiation, keeping the surface warmer than it would be under a clear sky. Without cooling to the dew point, water vapour does not condense into dew.
Common mistakes
- Reversing the cause - many students think cloudy nights are colder. Actually clouds INSULATE the surface.
- Confusing dew (surface condensation) with fog (air condensation) and frost (freezing dew).
- Not linking this to the same physics that keeps overcast winter nights warmer than clear ones.
Exam tip
Dew formation checklist: (i) Clear sky -> maximum longwave radiation escape -> surface cools. (ii) Calm wind -> no mixing, surface layer cools further. (iii) High humidity -> dew point reached easily. (iv) Cool surface below dew point -> condensation as dew. All four fail on cloudy, windy or dry nights.
Sources
- NCERT Class 11 - Fundamentals of Physical Geography (Atmospheric processes)
- Barry & Chorley - Atmosphere, Weather and Climate